Reduce logic to set and determine a diagram's KDChartModel considerably thanks to a comment from Sebastian.
This commit makes sure that a diagram's model is always set by doing this directly in registerDiagram().

By doing this I noticed that the d->kd*DiagramModel variable is actually redundant, as we can always get the model using d->kd*Diagram->model() and since it's now made sure that the existence of a diagram implies the existance of its model and the other way around.